Those will probably be splined-split-shaft-fitted knobs. If they don't come off with a reasonable degree of pulling you might try gently heating them with a hair dryer for several minutes to get them thoroughly warm. This will serve to expand and soften the plastic sufficient for them to let go.
Then again it is possible that a PO had epoxyed them onto shafts that had lost one half of themselves--not uncommon with the split-sline type. Then you may be looking at new pots, knobs--or both....
As for schematics if you have a Sams for a similar and earlier model it would probably give you an idea of how things are on that chassis; while the outward appearance changed with the times Zenith didn't change the design on the innards of that table radio all that much over the years....